Security hardening and first-run/portability improvements

Security (P0):
- Remove committed session-secret default; auto-generate and persist a
  random secret to the config volume when SESSION_SECRET is unset
  (prevents forgeable session cookies / auth bypass).
- Validate playlist names to a safe charset and render sldl configs via
  literal Python substitution instead of sed (closes a command-injection
  and path-traversal path through playlist names).
- shlex-quote credential values written to shell-sourced env files, and
  strip newlines from values patched into .conf files.
- Render playlist .conf files 0600; warn at startup if the master key is
  co-located with the config volume; document keeping it separate.

Portability:
- Configurable timezone via TZ (default UTC) instead of hardcoded Edmonton.
- Remove personal defaults (navidrome user "andrew", ephemeral.club URLs).
- Ship generic example seeds; move the cross-album dedup keep-list and the
  legacy playlist import to editable config files; drop the personal
  _upgrade.csv.
- Generic VPN reference in docker-compose.snippet.yml.

First-run experience:
- Redirect to /setup instead of 500 when OIDC is unconfigured; surface a
  missing master key inline; entrypoint exits with an actionable message
  when the config folder is not writable.
- Add unauthenticated /health (JSON) and /setup (checklist) diagnostics.

Docs:
- Write docs/ARCHITECTURE.md and docs/MIGRATION.md (previously referenced
  but missing); expand README with ownership, backups, advanced settings,
  and migration guidance.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/templates/dashboard.html

@@ -1,6 +1,15 @@
 {% extends "base.html" %}
 {% block title %}Dashboard — alembic{% endblock %}
 {% block content %}
+{% if setup_warnings %}
+<div class="panel" style="border:1px solid #b91c1c;margin-bottom:1.5rem;">
+  <strong>Setup needs attention</strong>
+  <ul class="muted" style="margin:0.5rem 0 0;padding-left:1.2rem;">
+    {% for c in setup_warnings %}<li>{{ c.detail }}</li>{% endfor %}
+  </ul>
+  <p class="muted" style="margin:0.5rem 0 0;">Full checklist on the <a href="/setup">setup page</a>.</p>
+</div>
+{% endif %}
 <div class="page-header">
   <div>
     <h1>Welcome back, {{ user.name or user.email }}</h1>